C#学生遇到程序调试异常，求精通大神帮忙

using System;
using System.Collections.Generic;
using
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace 距离
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            
            int x1 = Convert.ToInt32(textBox2.Text);
            int y1 = Convert.ToInt32(textBox1.Text);
           int x2 = Convert.ToInt32(textBox3.Text);
            int y2 = Convert.ToInt32(textBox4.Text);
            Point p1 = new Point(x1, y1);
            Point p2 = new Point(x2, y2);
            label7.Text = p1.Distance(p2).ToString();

        }
        class Point
        {
            public int X, Y;
            public Point(int I, int J) { X = I; Y = J; }
            public double Distance(Point p)
            {
                return System.Math.Sqrt((this.X - p.X) * (this.X - p.X) + (this.Y - p.Y) * (this.Y - p.Y));
    }
        }
    }
}